In recent years, in optical pickup apparatuses, the wavelength of the laser light is becoming progressively shorter in the laser light source used as the light source for reproducing the information recorded in optical disks or for recording information in optical disks, and for example, laser light sources of wavelengths of 400 nm to 420 nm are being realized such as blue-violet semiconductor lasers, blue SHG laser using wavelength conversion of infrared laser source using the second harmonic wave, etc.
If these blue-violet laser light sources are used, in the case in which an objective lens with the same numerical aperture (NA) as a DVD (Digital Versatile Disk), for an optical disk with a diameter of 12 cm, recording of 5 GB to 20 GB of information is possible, and when the NA of the objective lens is increased to 0.85, for an optical disk with a diameter of 12 cm, recording of 23 GB to 25 GB of information becomes possible. In the following, in the present patent specification, optical disks and magneto-optical disks using a blue-violet laser light source are collectively called “High Density Optical Disks”.
By the way, two standards have been proposed for high density optical disks at present. One is the Blu-ray disk (hereinafter abbreviated as BD) which uses an objective lens of an NA of 0.85 and has a protective substrate thickness of 0.1 mm, and the other is the HD DVD (hereinafter abbreviated as HD) which uses an objective lens of an NA of 0.65 to 0.67 and has a protective substrate thickness of 0.6 mm. Further, at present, DVDs or CDs with various types of information recorded in them are being marketed. In view of this current state of affairs, optical pickup apparatuses that carry out recording and/or reproduction of information for different optical disks have been proposed in Patents Documents 1 and 2.
Patent Document 1: International disclosure No. 03/91764 pamphlet
Patent Document 2: Japanese Unexamined Patent Application Publication No. 2005-209299